A catch-all mailbox receives email messages sent to invalid addresses under the exact same domain. For instance, a message sent to the misspelled firstname.lastname@example.org will go to email@example.com if the catch-all feature has been enabled for the latter. Thus, you can receive emails from friends or clients who may have sent a message to your email address with a typo or to an obsolete one, which they may still have, but you have already deactivated. Just one mailbox per domain can be a catch-all one and email forwarding cannot be enabled for such a mailbox. This is due to the fact that at a certain moment you may start receiving spam emails in the catch-all mailbox and the forwarding restriction implies that the spam will not be re-sent to a 3rd-party mailbox.